Created attachment 8678136 [details] Screenshot showing the CPU usage Affected builds: - Firefox 42 beta 9 (hang) - latest Nightly (very low browser performance) Affected OS's: - Mac OS X 10.11 - Windows 10 64-bit STR: 1. Start Firefox 2. Visit http://www.addictinggames.com/ 3. Select a game and wait Expected results: Games are played with no issues without affecting Firefox performance. Actual results: Website affects Firefox performance on Nightly and hang Firefox 42 beta and lower. Notes: - This is not a recent regression, it affects Firefox 40.0.2 as well - Screenshot attached with the values CPU usage from Activity Monitor (AM). - After 2-3 minutes both processes appear in AM as Not Responding.